FlexiMusic Generator is a unique tool to generate musical and non-musical sounds. If you want to create unique pieces of music and exciting new sounds never before heard on this planet, then you'll love the FlexiMusic Generator.

Four Cycle Synth[windows]

Free

Four Cycle Synth generates short chippy, very messy sounding samples, which are more meant to be used as raw sounds, which you filter, and add effects to, than finished synth sounds.
Import : no | Export : WAV

Possibly one of the most useful application that a game developer could have. DrPetter has written a program which randomly generates a sound effect based on selected options and slider settings. The results can then be saved as .wav files, ready for use in any projects.

Slashstone SoundFX Builder is a sound effect generator, made to generate the simple sound effects. The tool is opensource, released under the terms of the MIT license, written in FreePascal+Lazarus.
Import : SSFX | Export : SSFX, WAV

Sound Effects Generator is an easy to use utility to create Windows .WAV sound effects files. You create a sound by simply setting amplitude and frequency points, selecting a waveform and setting optional echo and distortion levels. Your sound effects file can be between 20ms and 10s long. You can set an unlimited number of amplitude and frequency points and the frequencies can vary between 10Hz and 4.8KHz. There are 18 waveforms to choose from (such as a sine wave, saw tooth, square wave etc...) and the sound quality can be 11,025Hz, 22,050Hz or 44,100Hz.

Tone Generator is a sine wave generator, frequency generator and signal generator that lets you create audio test tones, sweeps or noise waveforms using your computer or a PDA handheld.

TS-404[windows]

Free

Not really dedicated to 8-bit sound, Transistor Synth 404 is a virtual analogue, multitimbral synthesizer for Windows. You can use it to create fat basslines, melodies or just noise in true TB-303 (ReBirth) fashion. A good way to understand and play with basic waveforms (sine, square, triangle...) ! The ZIP archive contains patterns and tutorials.Import : PAT | Export : PAT